Fuel cells are devices in which electrochemical reactions are used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the nature of the fuel cell. Organic materials, such as methanol or natural gas, are attractive fuel choices due to their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Most currently available fuel cells are reformer-based fuel cell systems. However, because fuel processing is complex, and requires expensive components, which occupy comparatively significant volume, the use of reformer based systems is presently limited to comparatively large, high power applications.
Direct oxidation fuel cell systems may be better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger scale applications. In fuel cells of interest here, a carbonaceous liquid fuel in an aqueous solution (typically aqueous methanol) is applied to the anode face of a membrane electrode assembly (MEA). The MEA contains a layer of membrane electrolye which may be a protonically conductive, but electronically non-conductive membrane (PCM or membrane electrolyte). Typically, a catalyst, which enables direct oxidation of the fuel on the anode aspect of the PCM, is disposed on the surface of the PCM (or is otherwise present in the anode chamber of the fuel cell). In the fuel oxidation process at the anode, the products are protons, electrons and carbon dioxide. Protons (from hydrogen in the fuel and water molecules involved in the anodic reaction) are separated from the electrons. The protons migrate through the PCM, which is impermeable to the electrons. The electrons travel through an external circuit, which includes the load, and are united with the protons and oxygen molecules in the cathodic reaction, thus providing electrical power from the fuel cell.
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, a mixture comprised predominantly of methanol and water is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. The fundamental reactions are the anodic oxidation of the methanol and water in the fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed at an acceptable rate (more specifically, slow oxidation of the fuel mixture will limit the cathodic generation of water, and vice versa).
Direct methanol fuel cells are being developed towards commercial production for use in portable electronic devices. Thus, the DMFC system, including the fuel cell and the other components should be fabricated using materials and processes that are compatible with appropriate form factors, and are cost effective in commercial manufacturing. Furthermore, the manufacturing process associated with a given system should not be prohibitive in terms of associated labor or manufacturing cost or difficulty.
Typical DMFC systems include a fuel source, fluid and effluent management and air management systems, and a direct oxidation fuel cell (“fuel cell”). The fuel cell typically consists of a housing, hardware for current collection and fuel and air distribution, and a membrane electrode assembly (“MEA”) disposed within the housing.
A typical MEA includes a centrally disposed, protonically conductive, electronically non-conductive membrane (“PCM”). One example of a commercially available PCM is NAFION® a registered trademark of E.I. Dupont de Nemours and Company, a cation exchange membrane comprised of polyperflourosulfonic acid, in a variety of thicknesses and equivalent weights. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. On either face of the catalyst coated PCM, the electrode assembly typically includes a diffusion layer. The diffusion layer on the anode side is employed to evenly distribute the liquid fuel mixture across the anode face of the PCM, while allowing the gaseous product of the reaction, typically carbon dioxide, to move away from the anode face of the PCM. In the case of the cathode side, a diffusion layer is used to achieve a fast supply and even distribution of gaseous oxygen across the cathode face of the PCM, while minimizing or eliminating the collection of liquid, typically water, on the cathode aspect of the PCM. Each of the anode and cathode diffusion layers also assist in the collection and conduction of electric current from the catalyzed PCM.
As noted, the MEA is formed of a centrally disposed PCM that is sandwiched between two catalyst layers. The catalyst layers of the MEA in some architectures can be arranged such that a gas diffusion layer (GDL) is adjacent the cathodic catalyst layer to allow oxygen to be transported to the cathode, and a liquid and gas diffusion layer (LDL/GDL) is adjacent the anodic catalyst layer to allow liquid fuel to be transported to the anode, and to allow carbon dioxide to travel away from the anode. Generally, the entire MEA is placed into a frame structure that both compresses the MEA and provides an electron path. Those skilled in the art will recognize that sealing and application of significant pressure can be accomplished in various ways, but these aspects conventionally involve relatively large fastening components, such as screws, nuts and the like. Alternatively, a frame may be insert molded around the MEA in such a fashion that it is supported and compression is applied to the MEA as set forth in co-owned U.S. application Ser. No. 10/650,424, filed Aug. 28, 2003 and entitled “METHOD OF MANUFACTURING A FUEL CELL ARRAY AND A RELATED ARRAY,” which is incorporated by reference in its entirety. Regardless of the means by which the MEA is to be incorporated into a fuel cell, it is critical that the components of the MEA be aligned properly, otherwise the performance of the MEA, the fuel cell and the fuel cell system will be compromised.
Typically, MEA fabrication requires that the PCM and diffusion layers are bonded to each other or otherwise in intimate contact with each other. This is presently achieved by applying heat and pressure in a hot pressing or lamination process. More specifically, during fuel cell construction, a membrane electrode assembly is formed which includes a catalyzed membrane and at least one diffusion layer, which are aligned properly and then bonded to each other in a hot press operation. This step is generally labor intensive, and is therefore expensive when applied to multiple MEAs in a serial fashion. It is further possible to bond the MEA components to each other in parallel if multiple diffusion layers are placed on a single, comparatively large sheet of the protonically conductive membrane where the active sites (those areas on the sheet to which a catalyst has been applied) are electrically isolated from each other, but are still part of a contiguous piece of a protonically conductive membrane. Also, the membrane separates opposing gas diffusion layers and electrical contact between such opposing gas diffusion layers is undesirable since it may cause a fuel cell or array to be short-circuited.
As noted above, most or all of the processes for forming a MEA are presently very labor intensive. In particular, the components are assembled by hand and moved from one manufacturing station to another in the same manner. Thus, there is a need for a process for manufacturing and assembling a fuel cell or a fuel cell array, which automates the handling of components of a fuel cell during manufacture thereof to allow mass manufacture of such fuel cells or fuel cell arrays. Further, there is a need for improving the reliability of fuel cells and fuel cell systems by minimizing the variability between MEAs.
It is thus an object of the present invention to provide a cost-effective, highly efficient process for manufacturing a fuel cell or fuel cell array that allows mass manufacture of a fuel cell. It is a further object of the invention to provide a fuel cell that has been produced by such a process.